<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>第5-用css来控制</title>
<style>
    div{
        /*width:300px;即没有宽度*/
       width:300px;
       height:200px;
        background:blue;
    }
    #main {
        background: green;
        /*float:left;*/
        float:left;
        width:700px;
       height:200px;
    }
    #footer {
        background: grey;
        /*float:right;*/
        width:700px;
        float:right;
    }
</style>
</head>
<body>
    <div id="header"></div>
    <div id="main"></div>
    <div id="footer"></div>
</body>
</html>